Earth's Systems / Biological processes / Terrestrial ecosystem functioning / Living soil system / Mineral weathering and parent material / Pyrite oxidation and acid sulfate weathering
Pyrite oxidation and acid sulfate weathering
Iron sulfide (pyrite) exposed to Oxygen by drainage or excavation oxidises to sulfuric acid, which dissolves clays and mobilises Al, until carbonate or exchangeable bases neutralise it. Describes the Fe3+ and Acidithiobacillus feedback that makes the reaction self-accelerating below pH 4.
Boundary: where this process starts
Starts with: Pyrite is exposed to Oxygen when drainage lowers the water table or sulfidic sediment is excavated
Boundary: where it ends
Ends after: Calcite and exchangeable bases neutralise acid (CaCO3 consumes 2 mol H+ per mol) and raise pH; Acid, Al and sulfate drain to ditches and estuaries and cause fish kills and metal release
Line thickness = size of impact (1 to 5)% on a line = chance a fork takes that branch📄 = pieces of evidence on a stepNegative effectLoops backHypothesisResource used up (arrow into the step)Resource used, not used upResource made or released (arrow out of the step)
What am I looking at? The concepts on every map
- Participant
- Someone or something that takes part in a process or influences it: a plant, a microbe, a river, a machine, a force. Participants are the rows of a map.
- Resource
- Something a step uses or uses up: water, oxygen, sunlight, a tool. Each step has an arrow down to the resources it uses.
- Step
- One thing that happens in a process. Every step has at least one participant and carries evidence.
- Decision fork
- A step where the process can go several ways. Every outcome is labeled with a percentage, and the percentages add up to 100%. It is not limited to yes or no.
- Loop
- A line that carries the process back to an earlier step. We record whether it reinforces or balances, how strong it is (weak, moderate, strong), and its gain (0 to 1): the share of the effect that returns on each pass.
- Size of impact
- Every line says how much the earlier step affects the next one (1 to 5), in which direction (+, -, 0), and why. Thicker lines mean bigger impact.
- Evidence
- The support for a claim: a source and what it shows. Every step, line, change and vote needs it. Click any step or line to read its evidence.
- Boundary
- Where a process starts and where it ends. Every process, and every subprocess, has a start and an end.
- Subprocess
- A step that is itself a whole process, with its own boundary, steps and lines. Click a step marked ↘ to open it. Subprocesses nest as deep as the evidence goes.
- Description
- A plain-language summary of what a process is and what it covers.
- Hypothesis
- A proposed new connection between two parts of the map that nobody has linked. It carries a prediction, a way to test it, and a count of papers on each topic and on both. It is an unverified idea until someone tests it.
Every step, with its evidence
The same information as the map, as a list you can search and share. Click a participant or resource to see everywhere it appears.
Pyrite is exposed to Oxygen when drainage lowers the water table or sulfidic sediment is excavated
Evidence (1) · added by Fern
- Dent & Pons (1995), A world perspective on acid sulphate soils, Geoderma 67:263-276: Acid sulfate soils form when sulfidic (pyritic) material is drained and aerated; pyrite is stable only while saturated. (confidence 0.8)
Oxygen oxidises pyrite: Fe2+ and sulfate are released, giving about 2 mol H+ per mol FeS2 before Fe2+ is oxidised
Evidence (1) · added by Fern
- Nordstrom (1982), SSSA Special Publication 10, pp. 37-56: FeS2 + 7/2 O2 + H2O gives Fe2+ + 2 SO4(2-) + 2 H+; the Fe2+ then oxidises and hydrolyses to give 2 more H+. (confidence 0.8)
⑂ Fork: does solution pH fall below about 4 (no carbonate to buffer)
Evidence (1) · added by Fern
- Dent & Pons (1995), Geoderma 67:263-276: Acid sulfate soils are defined by pH below about 4 after oxidation; soils with carbonate stay near pH 6-8. Probabilities are an estimate (40 to 60 percent range) for drained coastal sulfidic sediments. (confidence 0.45)
Fe2+ oxidation is rate-limiting; Acidithiobacillus ferrooxidans speeds it by orders of magnitude (about 10^5 to 10^6) below pH 3.5
Evidence (1) · added by Fern
- Singer & Stumm (1970), Acidic mine drainage: the rate-determining step, Science 167:1121-1123: Abiotic oxidation of Fe2+ is very slow at low pH; iron-oxidising bacteria accelerate it by several orders of magnitude. (confidence 0.8)
Fe3+ attacks pyrite directly, 10 to 100 times faster than Oxygen, making more Fe2+ and acid
Evidence (1) · added by Fern
- Moses, Nordstrom, Herman & Mills (1987), Aqueous pyrite oxidation by dissolved oxygen and by ferric iron, Geochim. Cosmochim. Acta 51:1561-1571: Ferric iron is a far faster pyrite oxidant than dissolved oxygen at low pH, and the cycle Fe2+ to Fe3+ to Fe2+ is self-propagating. (confidence 0.8)
Sulfuric acid dissolves clays and releases Al3+; jarosite forms as yellow mottles below about pH 3.7
Evidence (1) · added by Fern
- Van Breemen & Buurman (2002), Soil Formation, 2nd ed., Springer, chapter on acid sulphate soils: Jarosite KFe3(SO4)2(OH)6 is stable below pH of about 3.7 and marks acid sulfate soil horizons; clay dissolution releases Al. (confidence 0.7)
Calcite and exchangeable bases neutralise acid (CaCO3 consumes 2 mol H+ per mol) and raise pH
Evidence (1) · added by Fern
- Stumm & Morgan (1996), Aquatic Chemistry, 3rd ed., Wiley, carbonate buffer chapter: CaCO3 + 2H+ gives Ca2+ + CO2 + H2O, so carbonate neutralises acid stoichiometrically until exhausted. (confidence 0.8)
Acid, Al and sulfate drain to ditches and estuaries and cause fish kills and metal release
Evidence (1) · added by Fern
- Sammut, Melville, Callinan & Fraser (1996), Estuarine acidification and fish kills in the Richmond River, Estuarine, Coastal and Shelf Science 43: Drainage of sulfidic floodplain soils exported acid and aluminium pulses that caused fish kills in Australian coastal rivers. (confidence 0.7)